One gathers exercise stretches back (6)
I believe the answer is:
reaper
'one gathers' is the definition.
(I've seen this before)
'exercise stretches back' is the wordplay.
I cannot really understand how this works, but
'exercise' could be 'pe' (Physical Education) and 'pe' is found within the answer.
'back' could be 'rear' (something's rear is its back) and 'rear' is located in the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for reaper that I've seen before include "Death is described as a grim one" , "Farmworker" , "One harvesting crop" , "Wordsworth's was Solitary" , "Farm machinery" .)